The present invention relates to high frequency power supplies for use with luminous tubular glass signage of the type often found in connection with retail advertising and decorating. More particularly, the present invention is specifically designed to power luminous tube signage of either the neon or mercury gas variety or, as is often the practice, signs having luminous tube segments of both gas types.
Until the relatively recent development of high frequency power supply technology, luminous tube signs (generally referred to generically as "neon signs" regardless of the actual gas employed), were uniformly powered by relatively massive low frequency (e.g. 60 Hz) high-voltage transformers, such transformers being both large and heavy.
High frequency power supplies (of which the present specification relates) offer significant reductions in both size and weight as compared to this older low frequency transformer technology. But not unexpectedly, there are inevitable trade-offs--in the present case, the concomitant liabilities of "neon bubble formation" and "mercury atom migration", problems uniquely associated with high frequency excited luminous tubes.
By way of additional background it should be observed that "neon" is, in fact, a misnomer. As previously noted, mercury is an equally common gas used in so-called "neon" signage. In fact, neon is only used in those signs, or those portions of signs, in which the `warm` colors of red, orange, pink and some shades of purple are desired. Where `cool` colors are intended, e.g. blue, turquoise and white, mercury is employed.
The visible spectral radiation of mercury may be employed directly as the visible medium or, as commonly, the ultraviolet radiation of mercury may be used in an indirect manner to excite phosphor coatings as required to produce the desired colors. It is significant to the present invention that many signs employ both neon and mercury luminous tube segments. It is therefore necessary that the present high frequency supply properly excite luminous tubes of either or both gas types.
The difficulty imposed by the foregoing is that mercury and neon are very different elements and therefore impart correspondingly dissimilar demands on their associated high frequency power sources. Neon, for example, remains a gas at room temperature while mercury is a liquid of low vapor pressure. Neon is relatively inert and therefore does not form chemical compounds. Mercury, by contrast, is very reactive and may combine with oxygen in the air to form, for example, various solid oxides. Such inherent differences result in the unique problems of neon bubble formation and mercury gas migration, as discussed in more detail below, and the corresponding difficulty in designing a high frequency power supply suitable for use with both gas types.
The principal difficulty with high frequency excited mercury tubes is that of "mercury migration". Current flow in mercury tubes is defined principally by movement of positive mercury ions. These ions are attracted to the negative electrode at which point they are neutralized to become mercury atoms. In principle this mechanism of current flow and ion neutralization should pose no difficulty as the `alternating` nature of the high frequency supply guarantees that each of the opposed tube electrodes is, in turn, negative and therefore receives its `share` of mercury ions. No net accumulation of mercury ions should be anticipated at either electrode. The density and distribution of mercury ions and atoms throughout the tube should remain substantially uniform. This is, in fact, the case where mercury tubes are excited by conventional low frequency 60 Hz power sources.
In practice, however, the use of high frequency power sources has been observed to cause the slow migration of mercury ions and atoms to one end of the tube. And due to the low vapor pressure of mercury, the redistribution and equalization of the mercury atoms through Brownian motion cannot be assured. As a consequence, one end of the tube is eventually depleted of the mercury gas required for light production thereby causing that end to grow dark.
The causes and solutions to the migration problem in high frequency excited mercury tubes is, at least in part, understood. One known cause is that of an overall or residual direct current (DC) component through the tube. Unfortunately, as outlined below, such DC components are often deliberately introduced in connection with neon tube high frequency power supplies as a solution to the bubble formation problem common with neon gas signs. Here, then, is one example of the difficulty known to the art in providing a single high frequency power supply suitable for use with both neon and mercury gas tubes. The `cure` for the neon bubble problem--i.e. the introduction of a small DC component--assures the ultimate discoloration or darkening of any mercury tube connected thereto.
It has also been discovered that the excitation of a mercury tube with a pure alternating current (AC) waveform--i.e. one without any residual DC component--may still cause mercury migration in the event that such waveform exhibits any asymmetry. Although the average positive and negative tube currents may be the same (again, no DC component), where the respective positive and negative half-cycles are not substantially identical, non-linearities associated with gas ion transit times or other tube phenomena result in the migration of the mercury atoms therein. Again, the solution to the migration problem--the use of an absolutely symmetrical AC waveform--is precisely the waveform that assures the greatest production of objectionable bubbles in neon gas tubes.
As noted, neon and mercury are quite different gases. Neon does not suffer from the ion/atom migration problem and therefore there is no corresponding restriction against the use of DC or non-symmetric AC power supply waveforms. Neon, however, has its own unique problem of bubble formation. Indeed, as discussed in U.S. Pat. No. 4,862,042 to Herrick, this phenomenon is well known and, in the cited reference, the deliberate introduction of DC currents is exploited to produce certain selected visually desirable effects associated with bubble formation and controlled movement of the bubbles within the neon tube.
These effects, however, are of limited and special application. In connection with the fabrication of ordinary neon signs, the presence of neon bubbles disrupts the uniform bar appearance of the elongated neon tube and is considered highly undesirable. As noted above, applying either a small DC bias through the neon tube or a non-symmetric waveform will force the relatively rapid motion of the bubbles, in turn, causing the bubbles to disappear, at least as perceived by the human eye.
The present invention seeks to simultaneously eliminate both the mercury migration and neon bubble formation problems thereby resulting in a high frequency supply that may be interchangeably used with tubes of either construction or, more commonly, with signs having tube segments of both gas types.
More specifically, the present invention relies on the discovery that the respective problems exhibit dissimilar time constants, that is, mercury migration generally requires a period of hours if not weeks or months to develop while neon bubble formation occurs substantially instantaneously. Thus, the present invention seeks to produce a DC or asymmetrical component of sufficient duration to visually defeat bubble formation while simultaneously assuring no long-term DC or asymmetrical component.
Several embodiments are proposed. In one embodiment, a zero DC component non-symmetrical waveform is generated with the asymmetry of this waveform being automatically and periodically reversed. In this manner, the applied waveform remains continuously non-symmetrical thereby assuring bubble invisibility while the long-term symmetry afforded by the periodically reversing asymmetry minimizes or eliminates all mercury migration. The arrangement proposed achieves this result at minimal circuit complexity and expense, specifically, by causing the requisite reversal within the low voltage driver portion of the supply thereby eliminating any relays or other high voltage switching components.
In an alternative arrangement, a DC biased symmetrical AC waveform is proposed in which the sense or polarity of the DC bias is, again, reversed at an appropriate long-term periodic rate. In this manner, minimum mercury migration is assured through application of AC symmetry and zero net DC bias over the long-term. The preferred embodiment employs a square-wave reversal of the DC bias. Although other waveforms, such as sine waveforms, may be utilized, the present approach minimizes circuit complexity by avoiding the bulk and cost of, for example, additional 60 Hz transformers or windings and, further, provides better bubble elimination. In this latter connection, the zero-crossing points of non-square wave DC bias reversal sources define partial bubble formation regions with correspondingly poorer bubble suppression capabilities.
More specifically, the preferred arrangement seeks to employ the series current fed push-pull resonant oscillator which is well known in the fluorescent ballast industry. In the present application, the oscillator output incorporates a leakage reactance output step-up transformer which, in turn, drives the neon or mercury load.
Certain difficulties were encountered, however, when this supply was connected to neon tube loads. A parasitic low frequency oscillation was observed which, as best understood, was controlled by the ionization time constant of the neon gas in concert with the series current feed choke as coupled through the leakage output transformer.
This oscillation was observed to build in intensity, often causing an over-voltage failure of the switching oscillator transistors. A further and most annoying problem resulting from this low frequency parasitic oscillation was that of an audible power supply squeal.
The present invention therefore seeks to implement the low cost series current fed oscillator through employment of a novel parasitic oscillation suppression arrangement. In this arrangement, a second winding is positioned and coupled to the series current feed choke and energy, related only to the parasitic oscillation, is coupled, rectified, and returned to the DC power source in a manner that both suppresses the unwanted oscillation but without the normal power losses associated with known suppression schemes.
A further feature of the present reversing DC current migration/bubble elimination high frequency oscillator is that of the output DC current switching circuitry. While it is generally known that residual DC tube currents cause mercury migration, and that the reversal of such currents minimize this migration, known current reversing arrangements have not been totally satisfactory, either due to cost or circuit efficacy. As noted above, for example, use of a series connected 60 Hz transformer is not believed to fully quench bubble formation and, in any event, is contrary to the underlying objectives associated with high frequency power supplies in its re-introduction of a relatively bulky 60 Hz transformer.
With specific reference to the present invention, DC current reversal is achieved through the switching of a diode element in alternate polarities across a reactance element in series with the reactance transformer output. The diode serves to shunt the reactance for current flow through the secondary in one direction only thereby generating the previously noted DC off-set current. By reversing the polarity of the diode, a corresponding reverse in neon tube DC current results.
The present invention, however, avoids the complexity and costs associated with multiple switching devices and diode elements ordinarily required to implement the required reactance polarity switching. Instead, an arrangement of two FET devices provides both the switching and diode functions by advantageously employing an intrinsic diode defined within the FET structure when the FET is in the off condition. Thus each FET alternately performs a switching and a diode current shunting function thereby resulting in a high performance mercury migration elimination circuit of minimum cost, complexity, and of corresponding increased reliability.
It is therefore an object of the present invention to provide a high frequency power supply suitable for use with either neon and/or mercury luminous tubes. Such supply should eliminate or minimize the formation of visible bubbles in neon tube segments and the migration of gas atoms in mercury tube segments thereby providing a efficacious high frequency power source suitable for exciting composite neon/mercury gas signs for substantially unlimited time periods. A further and important object is that such supply must be cost effective and reliable and consequently should avoid the use of additional and bulky 60 Hz transformers or windings and/or high voltage relays or similar switching devices.
These and other objects will become apparent from the Drawings and the specification including the Description of the Preferred Embodiment that follow.